Cooking ovens such as convection ovens and hot-air-impingement ovens that cook foods with heat by forming a circulated current of hot air stream inside a cooking chamber in which the foods are placed, are well known and widely used. Published documents such as, to name a few, Japanese Utility Model Published No. H6-23841 and Japanese Patent Applications Laid-Open Nos. H9-145063, H11-166737, 2000-329351, and 2001-311518 disclose examples of hot-air-circulation cooking ovens. On the other hand, Japanese Patent Published No. H9-503334 discloses an example of a hot-air-impingement cooking oven. Cooking ovens that combine a hot air stream with microwave heating are also well known (see Japanese Patent Applications Laid-Open Nos. H9-145063, H11-166737, and 2001-311518).
Now, as the basis of the present invention, the construction of a hot-air-circulation cooking oven will be described with reference to FIGS. 15 to 17. FIG. 15 is a front view of the cooking oven, FIG. 16 is a vertical sectional view thereof, and FIG. 17 is a perspective view showing the construction of a microwave heating device. The cooking oven 1 has a cabinet in the shape of a rectangular parallelepiped. Inside the cabinet 10, there is formed a cooking chamber 11 in the shape of a rectangular parallelepiped. The top and bottom of the cooking chamber 11 are formed by a ceiling wall 12 and a floor wall 13, respectively. Of the four sides of the cooking chamber 11, three are formed by a rear inner wall 14, a left inner wall 15, and a right inner wall 16, respectively, and the fourth side consists of an freely openable door 17. The door 17 and all the walls of the cooking chamber 11 are heat-insulated.
The cooking chamber 11, which is enclosed from six sides by the walls and the door as described above, has the following interior dimensions: 230 mm high, 408 mm wide, and 345 mm deep. It should be understood that all the values given as dimensions, speeds, temperatures, and the like in the present specification are merely preferable examples and are not meant to limit the scope of the present invention in any way.
Outside the rear inner wall 14, there is installed a blower 20. The blower 20 has a centrifugal fan 22 arranged inside a fan casing 21. This centrifugal fan 22 is rotated in the forward and backward directions by a reversible-rotation motor, which will be described later. The fan casing 21 is of a type that branches into two directions, and has an upper discharge port 23 and a side discharge port 24. The upper discharge port 23 connects to an upper duct 25 provided outside the ceiling wall 12. The side discharge port 24 connects to a side duct 26 provided outside the left inner wall 15.
The upper duct 25 has an upper blowout port 30 open to the cooking chamber 11. The side duct 26 has a side blowout port 31 open to the cooking chamber 11. In the rear inner wall 14, there is formed a suction port 32 of the blower 20. The upper blowout port 30 consists of a group of small cylindrical holes each 11 mm across. The side blowout port 31 and the suction port 32 are each formed by a group of perforations each 5 mm across.
As shown in FIG. 16, in the upper duct 25 is provided an upper heater 40. In the side duct 26 is provided a side heater 41. Outside the right inner wall 16, there are arranged a microwave heating device 42 that assists the heating by the upper and side heaters 40 and 41 and a controller 43 that controls the operation of the cooking oven 1 as a whole. On the outer front surface of the right inner wall 16, there is provided an operation panel 44 (see FIG. 15) that accepts instructions for the controller 43.
On the floor wall 13, there is arranged a turntable 50 on which to place foods. On the turntable 50 is placed a supporting means such as a grill or rack that suits the kind of food placed. Reference number 51 represents a turntable drive motor.
Outside the cooking chamber 11, there are arranged components as shown in FIG. 17. The microwave heating device 42, of which the existence is only abstractly illustrated in FIG. 16, is illustrated as a concrete component in FIG. 17.
The core component of the microwave heating device 42 is a microwave generating device 70. The microwave generating device 70 is realized with a magnetron, which is oscillated by a high-voltage transformer 71. The microwave generated by the microwave generating device 70 is fed by way of a waveguide 72 to a side wall of the cooking chamber 11, and is then discharged from a wave feed port 73 into the cooking chamber 11. For the microwave generating device 70 is provided a cooling fan 74. For the high-voltage transformer 71 is provided a cooling fan 75. On the back-face side of the cooking chamber 11, there is arranged a reversible-rotation motor 80 for rotating the centrifugal fan 22 in the forward or backward direction.
The cooking oven 1 operates as follows. First, the door 17 is opened. Then, among different types of supporting means such as grills and racks, one that suits the intended kind of food is placed on the turntable 50. On this supporting means, foods are placed directly or using a container. Then, the door 17 is closed.
After the door 17 is closed, cooking conditions are entered via the operation panel 44. Based on the thus entered cooking conditions, the controller 43 selects the optimum among a plurality of pre-programmed cooking methods. The controller 43 then drives the blower 20, upper heater 40, side heater 41, microwave heating device 42, and turntable drive motor 51 to start cooking.
For example, in a case where roasted chicken is prepared, a grill is placed on the turntable 50, and a chunk of meat is placed on the grill. Then, the door 17 is closed, and then, from the menu displayed on the operation panel 44, “roasted chicken” is selected. Now, the controller 43 operates the blower 20, upper heater 40, side heater 41, microwave heating device 42, and turntable drive motor 51 in a mode for preparing “roasted chicken.”
The upper heater 40 has a power rating of 1,700 W, and the side heater 41 has a power rating of 1,200 W. Out from each of the upper blowout port 30 and the side blowout port 31 blows a hot air stream having a temperature of 300° C. or more as measured at those ports. The controller 43 controls the blower 20 in such a way that the air stream blown out from the upper blowout port 30 has a air stream speed of 65 km/h or more, and that the air stream blown out from the side blowout port 31 has a air stream speed of 30 km/h or less. The turntable 50 is rotated at a rotation rate of 6 rpm.
In the case described above, cooking is achieved by a hot-air-impingement method whereby a high-speed hot air stream is blown onto the foods. This permits fast cooking of the chunk of meat. The temperature inside the cooking chamber 11 is automatically adjusted at the target temperature entered via the operation panel 44. The upper limit of the target temperature is 300° C.
Next, how sponge cake is prepared will be described. A rack is placed on the turntable 50. Then, dough to be cooked into sponge cake is placed on the turntable 50 and also on the rack. The door 17 is closed, and, from the menu displayed on the operation panel 44, “sponge cake” is selected. Now, the controller 43 operates the blower 20, upper heater 40, side heater 41, microwave heating device 42, and turntable drive motor 51 in a mode for preparing “sponge cake.” Also here, the turntable 50 is rotated at a rotation rate of 6 rpm.
Here, however, the controller 43 controls the blower 20 in such a way that a hot air stream having a air stream speed of 30 km/h or less blows out from the upper blowout port 30, and that a hot air stream having a air stream speed of 40 km/h or less blows out from the side blowout port 31. In this case, cooking is achieved by two-stage hot-air-circulation method, and this permits the dough placed on the turntable 50 and on the rack to be each cooked into fluffy sponge cake. The hot air stream that blows from above has a low speed, and thus does not deform by its pressure the dough in the process of rising.
In cooking, a hot air stream or a microwave may be used singly, or they may be generated simultaneously so that heating is achieved by their combined effect. Whether to use the effect of a hot air stream or a microwave alone or their combined effect is determined by a cooking program or through selection by the user.
The cooking oven 1 described above can cope with various kinds of food and various methods of cooking by adjusting the ratio of the volumes of air stream blown out by the blower 20, the volumes of air stream themselves, and the air stream speeds, and by adjusting the amounts of heat generated by the upper and side heaters 40 and 41 and the output of the microwave heating device 42.
The cooking oven 1 described above blows a hot air stream onto foods 60 from above as shown in FIG. 18, and blows a hot air stream onto it also from a side as shown in FIG. 19. In a case where, as shown in FIGS. 18 and 19, a grill 61 is placed on the turntable 50 so that foods 60 are held up in the air, to heat the bottom face of foods sufficiently, it is essential that a hot air stream be blown from a side. However, blowing out hot air streams simultaneously in vertical and horizontal directions causes the following problem.
By design, the hot air stream that is blown out in the horizontal direction from the side blowout port 31 is expected to form a powerful air stream that blows through up to the suction port 32 as indicated by arrow W in FIG. 20. This permits a sufficient amount of heat to be transmitted to the bottom face of the foods 60. Here, however, when a hot air stream is also blowing out in the vertical direction from the upper blowout port 30, it deflects the hot air stream blown out in the horizontal direction from the side blowout port 31 and weakens the power of this air stream with which it blows through along the bottom face of the foods 60. This makes it hard to transmit a sufficient amount of heat to the bottom face of the foods 60. This tendency is more striking when cooking is performed by a hot-air-impingement method by using a hot air stream that blows down from above at a high speed.
When a hot air stream is blown out from the side blowout port 31 onto foods 60 while it is being rotated by the turntable 50, consideration needs to be given also to the following phenomenon. The part of the foods 60 located at the center of rotation of the turntable 50 receives the hot air stream all the time. By contrast, the part of the foods 60 located off the center of rotation receives less of the hot air stream when it happens to be located away from the position where it faces the side blowout port 31. This results in uneven cooking of the foods 60 from one part of it to another.
Moreover, with respect to the microwave heating device 42, the following problem arises. The wave feed port 73 is covered with a cover such as a punched metal sheet or metal mesh. If the wave feed port 73 is not located appropriately, this cover is sprinkled with oil and food fragments blown off from the foods by the hot air stream. As such pollutants accumulate on the surface of the cover, they may start fire or invite electrical discharge by the microwave.